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Znorg. Chem. 1993, 32, 247-248
Structure and Solution Properties of a Dimeric Tetrahedral Vanadium(V) Chloride Akoxide Complex Debbie C. Crans,' Robert A. Felty, Oren P. Anderson, and Mary M. Miller Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 Received July 20, 1992
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Although the literature is now replete with a number of structurally characterizedoxovanadium(V) derivativescontaining organic ligands,' only three compounds: two vanadium(V) siloxidesz and one vanadium( V) chloride diaryloxide, VOCIc"O (ULTRA) (ULTRA = ~,~'-CH~(~-CH~-~-C(CH~)~C~HZO)Z),~ contain tetrahedrally coordinated oxovanadium(V). Structural characterization of oxovanadium(V) alkoxides show such compounds have coordination numbers greater than 4. In the solid state, VO(OCH3)3 is polymeric; each vanadium atom has a coordination sphere containing six ligands as a result of bridging alkoxide g r o ~ p s . ~The compounds VO(OCHZCHZCI)~~ and V O ( ~ y c l u - C ~ H also ~ 0 )contain ~~ loosely bridging alkoxide ligands to form five-coordinatedimeric complexes. Recently we reported the structure of a five-coordinate oxovanadium(V) chloride dialkoxide compound with a 1,2-diol ligand [VOCl(OC(CH~)ZC(CH,)ZO)]~.~ The halves of the dimer are related by a center of inversion with one oxygen of each alkoxidepinacol moiety bridging between vanadium atoms. The formation of dimers due to alkoxide, aryloxide, or oxide bridges is a common feature in &6 CH oxovanadium(III), -(IV), and -(V) compound^.^-^ In contrast, the exact nature of the bridging of 1,2-diols in aqueous complexes of vanadium(V) nucleosides is less well characterized and evidence for alternative bridging structures has been pre~ented.~The 0 bridging of alkoxo oxygen atoms in vicinal diols to form fiveI 03 \ coordinate vanadium dimers is not the only possibility, although no other structural characterization of such compounds has been reported. We have found that an alternative mode of bridging in a dinuclear vanadium(V) complex results when ethylene glycol is used as a 1,2-diol ligand. Herein we report the preparation and characterization of this first tetrahedrally coordinated dimeric oxovanadium(V) chloride alkoxide compound. Under an atmosphere of dry argon, dropwise addition of VOCl3 (0.92 g, 5.3 mmol) to a CHZC12 solution of 1,2-bis(trimethylFigure 1. (A) ORTEP drawing of 1. Selected bond distances (A) and si1oxy)ethane (1.1 g, 5.3 mmol) at -60 OC in the dark produced angles (deg): VI411 = 2.207 (l), V1-01 = 1.578(2), V1-02 = 1.741 a dark red solution. The temperature of the solution was raised (2), V1-03 = 1.743 (2),0 2 4 1 = 1.422(2),0 3 4 2 = 1.433 (3), C1C2a = 1.505 (3); CIl-V1-01 = 108.3 (l), Cll-V1-02 = 113.0 (l), to 25 OC, and the solution was stirred for 12 h. During that time, 01-V1-02 107.1 (l),Cll-V1-03 110.2(l), 01-V1-03 107.9 a white solid precipitated from solution. The solid was isolated ( I ) , 02-V1-03 = 110.2 (l), V1-02-Cl = 134.0 (2), V1-03-CZ = by filtration and redissolved in 10% (v/v) hexane/CHzClz solution; 135.3 (2). (B) Side view of 1. the resultant mixture was then allowed to cool in a freezer at -20 OC overnight. Colorless crystals of [VOCI(OCHZCH~O)]Z (1) showing the conformation of the ring and the location of the C1 were obtained in 60%yield.lo*'l ORTEP drawings of 1areshown and oxo substituents. The geometry about each of the vanadium in Figure 1, and selected bond lengths and angles are given in the atoms is a slightly distorted tetrahedron with one oxo ligand, two caption. Figure 1B is a side view of the ten-membered ring alkoxide oxygen atoms, and a chloro ligand. The bond angles between the atoms attached to the vanadium center are within (1) Recent reviews: (a) Chen, Q.;Zubieta, J. Coord. Chem. Rev. 1992,114, 4O of idealized tetrahedral angles. The bond lengths of V - 0 107. (b) Rehder, D. Angew. Chem.. In?. Ed. Engl. 1991.30,148. (c) Klemperer, W. G.; Marquart, T. A,; Yaghi, 0. M. Angew. Chem., Inr. and V-CI are 1.578 (2) and 2.207 (1) A, respectively, which Ed. Engl. 1992,31,49. compare well to those observed for the pinacol dimer (1.576 (1) (2) Feher, F. J.; Walzer, J. F. Inorg. Chem. 1991, 30, 1689. and2.219 (1)A). ThelengthsoftheV-Obonds(l.741(2)and (3) Toscano, P.J.; Schermerhorn, E. J.; Dettelbacher, C.; Macherone, D.; Zubieta, J. J . Chem. SOC.,Chem. Commun. 1991, 933. 1.743 (2) A) are 0.03 and 0.22 A shorter than those observed for (4) Cauglin, C. N.;Smith, H. M.; Watenpaugh, K. Inorg. Chem. 1966,5, the pinacol dimer. These differences are not surprising given the 2131. greater strain and pentacoordination in the pinacol dimer. Much (5) Preibsch, W.; Rehder, D. Inorg. Chem. 1990, 29, 3013. (6) Hillerns, F.; Olbrich, F.; Behrens, U.;Rehder, D. Angew. Chem.. Inr. smaller differences are observed when the bond lengths of 1 are Ed. Engl. 1992,31,441. compared with those of VO(OSiPh3)3 (1.743 (2), 1.745 (2). and (7) Crans, D. C.; Felty, R. A.; Miller, M. M. J . Am. Chem. SOC.1991,113, 1.739 (3) A) and VOCl(ULTRA) (1.752 (4) and 1.734 (3) A), 265. (8) (a) Hambley, T. W.; Judd, R. J.; Lay, P. A. Inorg. Chem., 1992,31, indicating that the bond lengths of 1 fall within the range of 345. (b) Bottomley, F.; Magill, C. P.; White, P. S.J . Am. Chem. SOC. expected values according to previously reported results for 1989,111, 3070. (c) Djordjevic, C.; Lee, M.; Sinn, E. Inorg. Chem. vanadium(V) compounds with ligands in a tetrahedral geometry. 1989,28, 719. (d) Cotton, F. A.; Lewis, G. E.; Mott, G.N. Inorg. Chem., 1983,22,378. (e) Gambarotta, S.;Bolhuis, F. V.;Chiang, M. Although a tetrahedral array of ligands about thevanadium(V) Y. Inorg. Chem. 1987,26,4303.(0 Mazzanti, M.; Floriani,C.J. Chem. center was suspected on the basis of the slV NMR chemical shift, SOC.,Dalron Trans. 1989,1793. the dinuclear structure with tetrahedral vanadium(V) centers (9) Crans. D. C.; Harnung, S. E.: Larsen, E.; Shin, P. K.; Theisen, L. A,: Trabjerg, I. Acra Chem. Scand. 1991, 45,456. was not anticipated. A mononuclear complex would have
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248 Inorganic Chemistry, Vol. 32, No. 3, 1993
contained tetrahedral vanadium; however, geometrically nonrestricted bidentate diols do not readily form mononuclear compounds, despite efforts to generate t h e ~ e . ~ Nevertheless, J~J~ it is surprising that the ethylene glycol forms a ten-membered ring in a binuclear complex rather than a five-membered ring in a mononuclear complex. X-ray crystallographic studies of vanadate bound to uridine inside ribonuclease A have shown a five-membered ring in a mononuclear complex.14 Also, fivemembered-ring containing products are thought to form between vanadate and various diols in aqueous solution.15 Despite these findings, MCD and CD spectroscopic studiesof aqueous solutions do not support a five-membered ring in the major vanadate nucleoside complex, but rather suggest alternative structures in which the diol bridges across two vanadium centers similar to that observed in l.9 The previously characterized pinacol complex is also dinuclear; however, the vanadium is pentacoordinate because one of the alkoxide oxygen atoms bridges between the twovanadium atoms present in this complex, forming an internal four-membered ring.7 This four-membered V-0-V-0 ring formed is emerging as a common motif in vanadium(V) alkoxide compound^.^-^ In 1, the distance from the alkoxide oxygen 0 2 a to the other vanadium atom V1 is 3.92 A. This distance is too large to allow the bridging motif and results in a compound without a V-0-V-O ring. Although it is possible that 1 is merely the form of the compound that crystallizes under the conditions employed in this work, the discovery of 1lends support to proposed alternative bridging structural arrangements between vanadium(V) and 1,2-diols. Dissolution of thecolorless crystalline material 1 in chlorinated solvents is slow and results in a clear orange solution. The compound is not very soluble but, when dissolved, may in part convert to an alternativecompound that is moresolubleandorange in color. Variable-temperature IH and 51VN M R spectra reveal that the compound undergoes complex changes in solution. The 51VNMR spectrum shows the presence of two compounds a t ambient temperature and suggests that the two signals at -387 and -455 ppm correspond to species with the same number of vanadium atoms. The twospeciesobserved at 25 OC areconverted into three species at -50 "C: two minor resonances and one major resonance at -298 ( 2.5u(F0) observed. Structure solved by direct methods (SOLV) (full-matrix least-squares, 65 parameters total, data/ parameters = 18.3) weighted [w = (d(F) + gF?)-l, g = 1.2 X IO-'] least-squares refinement on F. H atoms in idealized positions (C-H = 0.96 A, U(H) = 1.2U,,,(C)). Non-H atoms refined with anisotropic thermal parameters. At convergence ( ( A / U ) ~=, ,0.04, ~ = 0.009 for last cycle) R = 0.032, R, = 0.076, S = 2.07, ( A P ) , , , ~=~ 0.54 e A-3, and (Ap),,, = -0.61 e A-I. Neutral atom scattering factors and anomalousdispersion correctionsused (International Tablesfor X-Ray Crystallography; Kynoch Press: Birmingham, U.K., 1974; Vol. IV); all calculations performed using the SHELXTL PLUS (UNIX) program library (Sheldrick, G. M. SHELTXTL PLUS (UNIX), Rev. 5 ; Siemens Analytical X-Ray Instruments, Inc.: Madison, WI, 1990). Crans, D. C.; Marshman, R. W.; Gottlieb, M. S.; Anderson, 0. P.; Miller, M. M. Inorg. Chem. 1992, 31, 4939. Crans, D. C.;Felty, R. A.; Eckert, H.; Das, N. Manuscript in preparation. (a) Alber, T.; Gilbert, W. A.; Ponzi, D. R.; Petsko, G. A. Ciba Found Symp. 1983,93,4. (b) Wlodawer, A,; Miller, M.; Sjblin, L. Proc. N a t f . Acad. Sci. U.S.A. 1983, 80. 3628. Gresser, M. J.; Tracey, A. S. J . Am. Chem. Soc. 1986, 108, 1935.
1,
Communications for the observed chemical shift and line width changes in the 5lV N M R spectrum. Since the original two resonances at -387 and -455 ppm are observed a t ambient temperature in the same ratio before and after the VT studies, these processes are all reversible. The minor -298 ppm resonance will be discussed in detail in a full paper. The signal a t -328 ppm is reminiscent of that found for the pinacol vanadate dimer a t -324 ppm and is likely due to a dimer containing pentacoordinatevanadiumin a similar bridging alkoxidestructure. A complex witha similar chemical shift (-326 ppm) a t ambient temperature forms between VOCI3 and another vicinal diol, cis- 1,2-cyclohexanediol. The major resonance at -482 ppm is presumably due to the dinuclear ten-membered ring observed in the solid state. This expectation is supported by the solid-state 51VNMR spectrum of compound 1giving a resonance a t -474 ppm.I3 A chemical shift of -477 ppm is found when VOC13 reacts with trans-1,2-cyclohexanediol.~6 The restricted geometry of this ligand dictates the conformations possible in the complex, and accordingly the trans-1,2-cyclohexanediol would form a bridging complex with pentacoordinate vanadium(V) only at the expense of significant angle strain. The ethylene glycol ligand in 1 is not subjected to such steric limitations and accordingly forms both complexes. The reasons for the preferences of 1 await further structural and spectroscopic studies. In general, the change from tetracoordination to higher coordination compounds is accompanied by a red shift of the first (low energy) absorption band.9 Compounds VO(OSiPh3)32and 1 contain tetrahedral vanadium centers and are colorless crystalline materials as long as water is excluded during synthesis and storage. A number of other vanadate triesters VO(OR)3 containing bulky alkoxide ligands (e.g. R = C(CH3)3, C(C6H5)3, or l-adamantyl)17 are also colorless and presumably fourcoordinate. These compounds remain colorless in solution and have constant 51V NMR chemical shifts5J7dq18J9at different concentrations. In triesters with small alkoxide ligands (e.g. R = CH3, CH2CH3, or CHzCH2Cl) the coordination sphere of the vanadium is expanded by the formation of bridging dimeric or oligomeric deri~atives.~-'~-'~These compounds are yellow, orange, or red and have 51VNMR chemical shifts that change with increasing concentration in solution. The monochloro-substituted pinacol vanadate dimer also contains bridging alkoxide moieties and is deep red. The coordination number of oxovanadium(V) alkoxides (VO(0R)j) or oxovanadium(V) chloride alkoxides (VOCl(OR)2), including the vicinal diols we have prepared, correlates well with the color of these compounds, either in the solid state or in solution. In summary, the complex formed between ethylene glycol and V0Cl3 [VOCl(OCH2CH20)]2 (1) has resulted in the first tetrahedrally coordinated vanadium(V) alkoxide derivative to be structurally characterized. The compound is dinuclear and consequently presents a new type of 1,2-diol bridging across two vanadium(V) atoms in oxovanadium(V) alkoxide systems. Acknowledgment. D.C.C. thanks the National Institutes of Health and the Lilly Research Laboratories for funding this research. We thank Robert W. Marshman for stimulating discussions. O.P.A. thanks the National Science Foundation for the purchase and upgrade of the Siemens R3m diffractometer and computing system. Supplementary Material Available: Complete tables of experimental crystallographic details, atomic coordinates, anisotropic thermal parameters of all non-hydrogen atoms, and bond lengths and angles for [VOCI(OCH2CH20)]2 (1) ( 4 pages). Ordering information is given on any current masthead page. (16) Viet, M. T. P.; Sharma, V.; Wuest, J . D. Inorg. Chem. 1991,30, 3026. (17) (a) Mittal, R. K.; Mehrotra, R. C. Z . Anorg. Al/g. Chem. 1964, 327. 31 I . (b) Witke, K.; Lachowicz, A.; Bruser, W.; Zeigan, D. Z . Anorg. A f f g . Chem. 1980, 465, 193. (c) Hillerns, F.; Rehder, D. Chem. Ber. 1991,124,2249. (d) Crans, D. C.;Chen, H.; Felty, R. A. J . A m . Chem. SOC.1992, 114, 4543. (18) Paulsen, K.; Rehder, D.; Thoennes, D. Z. Naturforsch.. A 1978, H A , 834. (19) Lachowicz, A.; Hobold, W.; Thiele, K.-H.2.Anorg. A / / g .Chem. 1975, 418, 65.
